This invention relates to systems and methods for playing multimedia content and more particularly to systems and methods for synchronizing digital audio playback to a variable rate asynchronous clock.
Systems have been in use for synchronizing multimedia playback of independent devices for some time now. Typically a clock source is distributed from a master clock to all slave devices. The slave devices extract playback position and rate information from the master clock to synchronize playback with the master. Common clock formats are Society of Motion Picture and Television Engineers (SMPTE) Time-Code, and Musical Instrument Digital Interface (MIDI) Time-Code (MTC). These clock formats specify a method of periodically transmitting the current playback location to a slave device.
For example, in video production environments it is common to synchronize the playback of a digital audio recorder with the playback of video from an independent video recording device. The video recording device could send its master clock signal to the audio recorder. In another application, a hard disk recorder may be synchronized to an external Musical Instrument Digital Interface (MIDI) sequencer or an analog playback device, such as a reel-to-reel multitrack audio recorder.
In the above applications the clock is typically fairly stable. For some other applications the clock rate and direction may fluctuate quite dramatically. For example, an audio scrubbing system can be implemented in which the playback of an audio track is synchronized with a user""s movement of an input device across a representation of the audio waveform or time-varying spectrum. The user can move the input device forward and backward over a portion of the graphical representation. The movement of the input device is translated into a clock specifying the playback position (media time) and playback rate.
When the slave device is playing back digital audio, the input clock is asynchronous to the sample clock on the audio system""s digital to analog converter (DAC) and can speed up, slow down, change directions, or even stop at any given time. When the clock speeds up the playback of the audio needs to speed up to maintain synchronization. Likewise, when the clock slows down the playback of the audio needs to slow down. Conventional systems do this using sample rate conversion which results in pitch shifting of the audio content thus reducing the intelligibility, fidelity, and enjoyment of the playback. If a clock is not very stable it may periodically speed up and slow down thus causing the audio system to speed up and slow down thus introducing pitch artifacts into the audio signal.
FIG. 1 illustrates a conventional system 100. System 100 is a digital audio playback system that can be synchronized to an external clock. It includes a digital audio data storage 110, a clock extraction component 112, a sample-rate converter 114, and an audio output unit 116 that contains the Digital to Analog Converter (DAC) 118 and the DAC sample clock 120.
To maintain synchronization between the input clock and the output audio a xe2x80x9clocate and chasexe2x80x9d technique is performed. Initially the clock extraction component extracts the current playback location and playback rate from the input clock. Then audio playback is started at the current located position, the audio is sample-rate converted to speed up or slow down playback relative to the audio system""s sample clock, and the audio is output though the audio system""s DAC. Then the clock extraction component continuously updates the current playback rate and uses the rate to adjust the amount of sample-rate conversion done. In detail the steps are as follows:
1. Extract the current playback position and playback rate from the input master clock. Send the current position to the Digital Audio Data Storage block and send the current rate to the Sample-Rate Converter.
2. A block of one or more Audio samples corresponding to the current playback position is sent from the Digital Audio Data Storage to the Sample-Rate Converter.
3. The Sample-Rate Converter changes the sample rate of the audio stream sent through it thus generating more samples to slow down playback or generating fewer samples to speed up playback. The rate is chosen appropriately based on the DAC output sample rate and the current rate that is extracted from the input clock.
4. The audio samples are output through the audio system""s DAC, now at the proper rate and location to be synchronized with the input clock signal.
5. This process is repeated as long as playback is desired.
What is needed is a system and methodology for providing pitch preserved audio playback which can be synchronized to a variable rate external clock signal.
According to one aspect of the invention, a system and methodology provides pitch preserved audio playback synchronized to a variable rate external clock signal. Pitch is preserved by using the phase vocoder to synthesize output audio blocks.
According to another aspect of the invention, synchronization is maintained by driving the analysis time of the phase vocoder with the current media playback time derived from the master clock.
According to a further aspect of the invention, the standard phase vocoder procedure is followed, using the analysis time from the previous phase vocoder iteration and the current analysis time to derive the input hop size.
Additional features and advantages of the invention will be apparent from the following detailed description and appended drawings.